1. Field of the Invention
The present invention relates to processes and systems for drilling a borehole, and more particularly, to processes and systems for drilling a borehole wherein the real-time specific drilling energies applied to the borehole are continually controlled to efficiently approximate and deliver the least amount of energy required to destroy and remove a given unit volume of rock without sacrificing rate of penetration.
2. Description of Related Art
In the production of fluid, from subterranean environs, a borehole may be drilled in a generally vertical, deviated or horizontal orientation so as to penetrate one or more subterranean locations of interest. Typically, a borehole may be drilled by using drill string which may be made up of tubulars secured together by any suitable means, such as mating threads, and a drill bit secured at or near one end of the drill string. Drilling operations may also include other equipment, for example hydraulic equipment, mud motors, rotary tables, whipstocks, as will be evident to the skilled artisan. Drilling fluid may be circulated via the drill string from pumps conjugate to the drilling rig through the drill bit. The drilling fluid may entrain and remove cuttings from rock face adjacent the drill bit and thereafter be circulated back to the drilling rig via the annulus between the drill string and borehole. After drilling, the borehole may be completed to permit production of fluid, such as hydrocarbons, from the subterranean environs.
As drilling a borehole is typically expensive, for example up to $500,000 per day, and time consuming, for example taking up to six months or longer to complete, increasing the efficiency of drilling a borehole to reduce cost and time to complete a drilling operation is important. Historically, drilling a borehole has proved to be difficult since an operator of the drilling rig typically does not have immediate access to, or the ability to make decisions based upon detailed rock mechanical properties and must rely on knowledge and experience to change those drilling parameters that are adjustable. Where a drilling operator has no previous experience in a given geological area, the operator must resort to trial and error to determine the most favorable settings for those adjustable drilling parameters. Processes have been proposed which utilize a traditional calculation of mechanical specific energy (MSE), which is the summed total of two quantities of energy delivered to the rock being drilled, torsional energy and gravitational energy, and manual adjustment of drilling parameters as a result of such calculation in an attempt to increase drilling efficiency. The original calculation developed by Teale, R. (1965) is as follows:MSE=(Wb/Ab)+((120*π*RPM*T)/(Ab*ROP))
Where:                MSE=Mechanical Specific Energy (psi)        Wb=weight on bit (pounds)        Ab=surface area of the bit face, or borehole area (in2)        RPM=revolutions per minute        T=torque (ft-lbf)        ROP=rate of penetration (ft/hr)        
The basis of MSE is that there is a measurable and calculable quantity of energy required to destroy a unit volume of rock. Operationally, this energy is delivered to the rock by rotating (torsional energy) and applying weight to (gravitational energy) a drill bit via the drill string. Historically, drilling efficiency could then be gauged by comparing the compressive strength of the rock against the quantity of energy used to destroy it. More recently, real-time monitoring of rock properties and calculation of MSE based upon such real time properties of drilling operations has been proposed to increase drilling efficiency by monitoring and responding to fluctuations in real-time MSE. However, a need still exists to improve the understanding and efficiency of the process of drilling a borehole.